Infections caused by Gram-negative bacteria may be complicated by shock, multi-organ failure and death. Since bacterial lipopolysaccharide (LPS) is considered an important initiator of sepsis, clinical trials have been conducted with agents designed to neutralize its biologic activity or to modify the cytokines induced by LPS. These studies reported a decreased mortality in those patients most ill, but an increased mortality in those least ill. This proposal addresses this apparent paradox. We suggest that the host response to LPS is involved not only in the development of sepsis, but also in host anti-bacterial defenses, and that interferon gamma (IFN gamma) plays a central role in both the beneficial and deleterious responses to LPS. To test this hypothesis, in Aim Ia we will determine if the induction of IFN gamma is essential to the efficacy of treatments (exogenous tumor necrosis factor alpha [TNF] and IL-1; BCG; or LPS) previously shown to restore or enhance anti-bacterial protection in mice. In Aim Ib we will examine whether a threshold exists at which the beneficial enhancement of antibacterial defenses by LPS stimulation gives way to lethal sepsis, and what role IFN gamma may play in this transition. We hypothesize that a synergistic interaction of IFN gamma with either TNF alpha or IL-1 occurs as lethal doses of LPS are approached. Using SCID mice deficient in B and T lymphocytes or monoclonal antibody (MAb) depletion of cells in normal mice, we will determine if NK and/or gamma delta T cells are important sources of IFN gamma in these early responses to LPS. Since host anti-bacterial defenses can be adoptively transferred to naive mice with spleen cells, in Aim II we will determine the mechanisms by which splenocytes generate LPS-stimulated anti-bacterial defenses. We hypothesize that LPS induces a paracrine loop between macrophages, NK and/or gamma delta T cells that leads to the rapid production of IFN gamma, and that IL-12, IL-15 and IL-18 regulate this early induction of IFN gamma. We will pre-treat mice in vivo or spleen cells in vitro with either LPS or cytokines, adoptively transfer spleen cell subpopulations to naive mice and determine their ability to resist lethal bacterial challenge. In Aim III we will determine the mechanisms by which the macrophage response to LPS-induced cytokines is converted from a limited, effective response that kills bacteria to a septic one capable of collateral tissue damage and/or host death. We postulate that IFN gamma stimulation enhances the magnitude and kinetics of LPS-induced early gene activation and TNF alpha secretion by macrophages through interactions of signal transduction molecules. The mechanisms of these synergistic interactions will be determined in a macrophage cell line and in macrophages from Stat1 knockout (IFN gamma non-responsive) and C3H/HeJ (LPS-hyporesponsive) mice.. The demonstration of a spectrum of responses to LPS in these studies may identify the earliest changes in cytokine responses that predict the development of sepsis.